Weather is the atmospheric conditions, along with short-term changes, of a certain place at a certain time. Weather Weather can change quickly or be the same for several days in a row. What is weather?

Temperature and rainfall are just two of the variables used to describe weather. Meteorologists use a variety of variables to predict weather, including air temperature, air pressure, wind speed and direction, humidity, cloud coverage, and precipitation. Weather Variables

Air temperature is the measure of the average kinetic energy of molecules in the air. Molecules in warm air move faster than molecules in cold air. Weather Variables (cont.)

Air pressure is the pressure that a column of air exerts on the air, or surface, below it. Air pressure Air pressure is measured with an instrument called a barometer. Weather Variables (cont.)

As air moves from areas of high pressure to areas of low pressure, it creates wind. Meteorologists measure wind speed using an instrument called an anemometer. Weather Variables (cont.)

The amount of water vapor in the air is called humidity.humidity When the humidity is high, there is more water vapor in the air. The amount of water vapor present in the air compared to the maximum amount of water vapor the air could contain at the temperature is called relative humidity.relative humidity Weather Variables (cont.)

The dew point is the temperature at which air is fully saturated because of decreasing temperatures while holding the amount of moisture constant.dew point Weather Variables (cont.)

Clouds are water droplets or ice crystals suspended in the atmosphere. A cloud that forms near Earth’s surface is called fog. Weather Variables (cont.)

PrecipitationPrecipitation is water, in liquid or solid form, that falls from the atmosphere. Weather Variables (cont.)

Rain, snow, sleet, and hail are forms of precipitation.

Rain is precipitation that reaches Earth’s surface as droplets of water. Snow is precipitation that reaches Earth’s surface as solid, frozen crystals of water. Weather Variables (cont.)

The water cycle is the natural process in which water continually moves among oceans, land, and the atmosphere.water cycle Weather Variables (cont.)

A high-pressure system is a large body of circulating air with high pressure at its center and lower pressure outside of the system.high-pressure system Pressure Systems

A low-pressure system is a large body of circulating air with low pressure at its center and higher pressure outside of the system.low-pressure system Pressure Systems (cont.)

Air masses are large bodies of air with distinct temperature and moisture characteristics. Air masses An air mass forms when a large high pressure system lingers over an area for several days. As a high pressure system comes in contact with Earth, the air in the system takes on the temperature and moisture characteristics of the surface below it. Air Masses

Five main air masses impact climate across North America. Air Masses (cont.)

Air masses are classified by their temperature and moisture characteristics. Continental air masses form over land. Maritime masses form over water. Air Masses (cont.)

Tropical air masses form in the warm equatorial regions. Polar air masses form over cold regions. Arctic and Antarctic air masses form near the poles, over the coldest regions of the globe. Air Masses (cont.)

A weather front is a boundary between two air masses.front Changes in temperature, humidity, clouds, wind, and precipitation are common at fronts. Fronts

When a colder air mass moves toward a warmer air mass, a cold front forms. Brings gusty winds, lower temperatures, heavy rain or hail, and severe weather. Fronts (cont.)

A warm front forms when lighter, warmer air moves toward colder, heavier air. Brings gentle rain or snow then warmer temperatures. Fronts (cont.)

When the boundary between two air masses stalls, the front is called a stationary front. Usually brings several days of cloudy wet weather. Fronts (cont.)

When a fast-moving cold front catches up with a slow-moving warm front, an occluded or blocked front forms. Usually brings cool temperatures and lots of rain or snow. Fronts (cont.)

A surface report describes a set of weather measurements made on Earth’s surface.surface report An upper-air report describes wind, temperature, and humidity conditions above Earth’s surface.upper-air report Radar measures precipitation using radio waves that bounce off raindrops and snowflakes. Measuring the Weather

Doppler radarDoppler radar is a specialized type of radar that can detect precipitation as well as the movement of small particles, which can be used to approximate wind speed. Measuring the Weather (cont.)

Weather maps contain symbols that provide information about the weather.

The station model displays data from many different weather measurements for a particular location.

A station model uses observations from surface reports and upper-air reports. Isobars are lines that connect all places on a map where pressure has the same value. Isobars Isobars show the location of high- and low-pressure systems and provide information about wind speed. Weather Maps

Isotherms are lines that connect places with the same temperature. They show which areas are warm and which are cold. Weather Maps (cont.)

Computer models are detailed computer programs that solve a set of complex mathematical formulas. Computer models Computer models are used in modern weather forecasts to predict what temperatures and winds might occur, when and where it will rain and snow, and what types of clouds will form. Predicting the Weather

Predicting the Weather (cont.) Government meteorological officers also use computers and the Internet to exchange weather measurements continuously throughout the day.

Thunderstorms have warm temperatures, moisture, and rising air, which may be supplied by a low- pressure system. Thunderstorms have a three-stage life cycle: the cumulus stage, the mature stage, and the dissipation stage. Severe Weather

The cumulus stage is dominated by cloud formation and updrafts. After the cumulus stage, downdrafts, which are air currents moving vertically toward the ground, begin to form. Severe Weather (cont.)

In the mature stage, heavy winds, rain, and lightning dominate the area. In the dissipation stage, updrafts stop, winds die down, lighting ceases, and precipitation weakens. Severe Weather (cont.)

A tornado is a violent, whirling column of air in contact with the ground.tornado Tornadoes form when thunderstorm updrafts begin to rotate. Tornado Alley is the name given to the area of the central United States that experiences the most tornadoes. Severe Weather (cont.)

An intense tropical storm with winds exceeding 119 km/h is a hurricane.hurricane Severe Weather (cont.)

A blizzard is a violent winter storm characterized by freezing temperatures, strong winds, and blowing snow.blizzard Severe Weather (cont.)

The U.S. National Weather Service issues watches and warning for different types of severe weather. A waatch means that severe weather is possible. A warning means that severe weather is already occurring. Severe Weather (cont.)